A cathode ray tube may be permanently damaged if, during the operation thereof, the intensity of the spot formed on the cathode ray tube face is either too great or stays in one position for too long a period of time. It is possible for this damage to occur by virtue of failure of the deflection system of the cathode ray tube causing the electron beam to stay in one position for too long a period of time and burning a spot in the phosphor on the face of the tube.
It is recognized that the internal face of the cathode tubes are normally coated with phosphorescent materials which scintillate upon being impacted by the electrons of the electron beam. This phosphorescent material then scintillates causing light photons to be released which produces the "writing" on the face of the cathode ray tube.
It is well known that if the energy level of the electron beam reaches a certain intensity level, the scintillating characteristics of the phosphorescent material can be permanently damaged. This permanent damage is often observed as a "burn mark" on the face of the electron tube. This "burn mark" is produced by the failure of the phosphorescent material to scintillate when subsequently swept by the electron beam after having been onced damaged.
We have determined that "burns" may occur on the face of a cathode ray tube either over a short or long period of time. The burns occurring from a short period of time have been observed through such as the previously-mentioned failure of the cathode ray tube deflection system or by the beam essentially stopped scanning and impacted a given spot area on the cathode ray tube. Burns may additionally be caused in a cathode ray tube if the deflection rate of the beam is sufficiently slowed and the intensity of the beam itself is sufficiently high. It should be recognized that burns in the face of the cathode ray tube occur because of the exceeding of acceptable limits of phosphor on a given point of its tolerance to electron bombardment. It is thus recognized that a less intense beam, however stationary, may cause a burn equally as permanent as a high intensity beam being scanned at a slow rate of speed. Should the per-unit radiation of a given defined spot of phosphor exceed the tolerance limit of phosphor, the burn occurs, whether the beam is stationary or scanning.
High voltage shut-down circuits for cathode ray tubes are known in the art. For example, U.S. Pat. No. 4,045,742 issued Aug. 30, 1977 describes means for shutting down a cathode ray tube responsive either to excess of high voltage or beam current. Additionally, U.S. Pat. No. 3,546,536 issued Dec. 8, 1970 illustrates an alternative mechanism for shutting down a cathode ray tube (television set) when the voltage to the picture tube exceeds a predetermined level. In this instance, one at which excess X-radiation may eminate from the television receiver. U.S. Pat. No. 3,813,580 issued May 28, 1974 illustrates a high voltage protection circuit for a television receiver to blank the electron beam of a picture tube when the accelerating voltage to the cathode ray tube or picture tube exceeds a predetermined level.
The present invention has particular application to the field of scientific instruments wherein a cathode ray tube is used to portray an image of various information generated by the scientific instrument. In the case of the present embodiment, the cathode ray tube is the viewing tube of a scanning electron microscope wherein the beam of the cathode ray tube is synchronized with the beam of the electron microscope as it scans across the image area of a specimen. In the present described embodiment, the cathode ray tube is a high resolution tube having thereon approximately 4,000 lines and a special long-persistance phosphor. It should be recognized that such instruments may be particularly susceptible to burns by a beam if the scanning rate of the electron beam becomes too slow or the intensity of the beam itself as, for instance, in response to signals generated from the specimen surface, becomes too intense. It should also be recognized that damage to the cathode ray tube, instead of being in terms of a few hundred dollars for the replacement of the tube, may in such high resolution devices run to a few thousand dollars.
It should, therefore, be evident that apparatus within the scanning circuits of the cathode ray tube which prevent the possibility of the burning of the phosphor of the cathode ray tube, perform a useful and important function.